Multi-layer via-less thin film resistor

ABSTRACT

The present disclosure is directed to a thin film resistor having a first resistor layer having a first temperature coefficient of resistance and a second resistor layer on the first resistor layer, the second resistor layer having a second temperature coefficient of resistance different from the first temperature coefficient of resistance. The first temperature coefficient of resistance may be positive while the second temperature coefficient of resistance is negative. The first resistor layer may have a thickness in the range of 50 and 150 angstroms and the second resistor layer may have a thickness in the range of 20 and 50 angstroms.

BACKGROUND

1. Technical Field

The present disclosure is directed to thin film resistors, and moreparticularly, to multi layer thin film resistor structure that laterallyconnects first conductive layers of adjacent interconnects.

2. Description of the Related Art

Precision resistors provide stable resistances for integrated circuitsused in various precision electronic devices, such as pacemakers,printers, and testing or measuring instruments. Each electronic deviceutilizes specific resistance values and operates in differentconditions. Manufacturers tailor precise resistance values for eachelectronic device by controlling the size of the resistor and by usingmaterials having low temperature and voltage coefficients. However, theperformance of these precision resistors is often impacted by variationsin operating conditions, like temperature and voltage. Manufacturersstrive to achieve tight tolerances with respect to resistance and sizeto better attain precise, stable resistances.

Conventional precision resistors include diffused resistors and lasertrimmed polysilicon resistors. Diffused resistors have a dopantintroduced into a polysilicon resistor layer in the substrate, forming adoped active region, such as a P-well or P-body in the substrate.High-ohmic polysilicon resistors have temperature coefficients ofresistance in the range of 1,000 and 3,000 parts per million per degreeCelsius with resistances in the range of 1 k and 10 k ohms/square. Inaddition, the resistance of a doped polysilicon layer changes withtemperature because the carriers are activated, which can causeperformance drifts that follow the operating temperature.

A length and width of the doped resistor layer, a depth of diffusion,and a resistivity of the dopant control the specific resistanceachieved. Junction isolation techniques isolate the diffused resistorfrom other elements in the substrate. These isolation techniques, whichtake up precious space on the substrate, minimize the adverse impacts ofspace charge effects of p-n junctions that can alter the resistance asthe operating voltage and frequency change. To compensate for thesechanges in resistance, manufacturers often include additional circuitryadjacent the resistor, thereby using more substrate area surrounding theresistor.

Laser trimming removes or cuts away portions of a polysilicon resistorlayer to increase the resistance. More particularly, the laser altersthe shape of the resistor to achieve a desired resistance value. As withdiffused resistors, laser trimmed resistors use large areas of thesubstrate in order to achieve precise resistor values. The large areadimensions also allow these resistors to dissipate heat to thesubstrate. The size requirements of these resistors impact the densityof devices in the integrated circuit. As a result of the continuedminiaturization of integrated circuits, manufacturers strive to reducethe space requirements of precision resistors.

In addition to the horizontal space requirements, these precisionresistors impact vertical space requirements of the associatedelectronic device. For example, FIG. 1 is a known electronic device 10having a precision resistor 12 connected to an upper metal layer 14through a plurality of vias 16 as disclosed in U.S. Pat. No. 7,410,879to Hill et al. The electronic device includes a first metal layer 18formed on a substrate 20. The precision resistor is formed on a firstdielectric layer 22 that overlies the first metal layer 18 and thesubstrate 20. Prior to forming the vias 16, a resistor head contactstructure 24 is formed over ends 26 of the precision resistor 12. Theresistor head contact structure 24 includes a titanium tungsten layer 28and a second dielectric layer 30.

A thin film resistor layer is generally evaporated or sputtered on thesubstrate 20 and then patterned and etched to form the resistor 12. Inorder to operate, the resistor requires an electrical connection to bemade to the ends 26, which requires two mask layers, one to shape theresistor 12 and one to form the resistor head contact structures 24.These resistor head contact structures 24 protect the resistor duringthe via etch that will electrically connect upper metal layer 14 to theresistor 12.

A third dielectric layer 32 is formed overlying the precision resistor12, the resistor head contact structure 24, and the first dielectriclayer 22. The plurality of vias 16 are formed through the thirddielectric layer 32 and filled with a conductive material toelectrically connect the precision resistor 12 to the upper metal layer14. Having the precision resistor 12 separated from the first metallayer 18 by the first dielectric layer 22 and having the precisionresistor 12 separated from the upper metal layer 14 limits themanufacturer's ability to reduce the size of the electronic device. Moreparticularly, having the first metal layer 18 and the resistor 12separated by the first dielectric layer 22 adds significant verticaldimensions to the electronic device 10.

FIG. 2 is an isometric view of a known technique for forming precisionresistors without vias connecting upper metal layers to the resistors.An electronic device 40 has a tantalum nitride resistor 42 formeddirectly on exposed portions of an aluminum layer 44 and on a planarizeddielectric layer 46 as disclosed in U.S. Pat. No. 5,485,138 to Morris.The aluminum layer 44 is formed on a lower level dielectric layer 48formed on a gallium-arsenide substrate 50.

The process of forming the resistor 42 includes depositing the aluminumlayer 44 directly on the lower level dielectric layer 48 and thenpatterning and etching the aluminum to form metal lines. The dielectriclayer 46 is then formed over the aluminum layer 44. A planarization stepsmoothes a top surface of the dielectric layer 46. Subsequently, an areaof between 1 and 1,000 angstroms of the top of the aluminum layer 44 isexposed. A tantalum nitride layer is then deposited and etched to formthe tantalum nitride resistor 42. As can be clearly seen in FIG. 2, theresistor 42 is significantly larger than the aluminum layer 44, whichadds additional vertical dimensions to the electronic device 40.

Thin film resistors are attractive for high precision analog and mixedsignal applications that have size constraints. Thin-film resistors aregenerally more precise than diffused and laser trimmed polysiliconresistors. Several parameters define performance of thin film resistorsincluding the resistor's value, the resistor's tolerance, and thetemperature coefficient of resistance. The temperature coefficient ofresistance provides an adequate means to measure the performance of aresistor. Thin film resistors have superior temperature coefficient ofresistance and voltage coefficients of resistance, i.e., a low thermalcoefficient of resistance and a low voltage coefficient of resistance.Thin film resistors also have good resistor matching and stability underthermal stress for use in integrated circuits to implement a specificfunctionality, including biasing of active devices, serving as voltagedividers, and assisting in impedance matching, to name a few.

Many electronic devices utilize high precision thin film resistors, suchas operational amplifiers, digital-to-analog converters with highaccuracy, implanted medical devices, and radio frequency circuits withhigh accuracy. Radio frequency (RF) circuits utilize thin film resistorsfor input/output circuitry in both radio frequency complementarymetal-oxide semiconductors (CMOS) and RF silicon germanium technology.In these high precision applications, thin film resistors with a hightolerance, good linearity, a low temperature coefficient of resistance,a high quality factor, and reliability in high current applications aredesired. These precision resistors should have a sheet resistancebetween 100 and 2,000 ohms/square with a temperature coefficient ofresistance between −100 and +100 parts per million per degree Celsius.However, integrating thin film resistors into existing product lines canbe difficult due to the reduced size of many electronic devices.

BRIEF SUMMARY

The present disclosure describes a thin film resistor that laterallyconnects adjacent interconnect structures in an integrated circuit. Eachinterconnect structure includes a first conductor and a secondconductor. A thin film resistor layer is formed over the interconnectstructures and directly connects sidewalls of the first conductors toeach other. The thin film resistor layer is also over the substrateextending between the interconnect structures. A portion of the thinfilm resistor layer is covered by a photoresist, leaving the remainderof the integrated circuit uncovered and exposed to an etch. After theetch, the photoresist is removed and the thin film resistor remains.

The thin film resistor may include a plurality of resistive layers, suchas two chromium silicon layers, each between 50 and 500 angstroms inthickness. Alternatively, the thin film resistor may include multipledifferent resistive layers having different chemical compositions anddifferent resistance values.

The thin film resistor may be covered by a dielectric cap, such as asilicon nitride layer between 400 and 600 angstroms in thickness. Thedielectric cap provides the thin film resistor with good stability andtemperature characteristics while protecting the thin film resistorlayer from subsequent processing steps. The thin film resistor layer isvery sensitive to damage from plasma etches, and if not protected suchdamage can impact the sheet resistance and temperature coefficient ofresistance. The dielectric layer can act as a heat sink to dissipateheat away from the thin film resistor.

Laterally connecting adjacent interconnect structures through the firstconductor eliminates the need for the processing steps of forming theresistor head contact structures and the vias that connect the resistorto the next metal level as described above with respect to the priorart. The thin film resistor provides thermally stable thin filmresistors capable of achieving precise resistance values in a smallarea.

The interconnect structures may include a protective coating thatprevents the etch from damaging the second conductor during removal ofthe excess thin film resistor layer. The protective coating reduces theconcerns raised by misalignment of the photoresist because even if thereis misalignment, the protective coating prevents the etch from damagingthe interconnect structures. This allows for minimum spacing betweeninterconnect structures to be achieved.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing and other features and advantages of the presentdisclosure will be more readily appreciated as the same become betterunderstood from the following detailed description when taken inconjunction with the accompanying drawings.

FIG. 1 is a side view of a known precision resistor having vias toconnect metal layers taken through a semiconductor device;

FIG. 2 is an isometric view of a known precision resistor directlycontacting a top surface of an aluminum line over a semiconductorsubstrate;

FIG. 3 is a cross-sectional view of an integrated circuit having a thinfilm resistor laterally connecting adjacent interconnect structures;

FIG. 4 is a cross-sectional view of the integrated circuit of FIG. 3having a dielectric cap over the thin film resistor;

FIG. 5 is a cross-sectional view of the integrated circuit of FIG. 3having a plurality of resistive layers covered by a dielectric cap;

FIGS. 6-11 are cross-sectional views of various steps in the process toform an integrated circuit having interconnect structures and a thinfilm resistor;

FIGS. 12 and 13 are cross-sectional views of an alternative process toform interconnect structures and a thin film resistor;

FIG. 14 is a cross-sectional view of an interconnect structure having aplurality of thin film resistor layers covered by a dielectric cap;

FIGS. 15-18 are cross-sectional views of yet another alternative processto form interconnect structures and a thin film resistor;

FIG. 19 is a cross-sectional view of a partially formed integratedcircuit having a thin film resistor formed in a recess in a substratebetween interconnect structures;

FIG. 20 is a cross-sectional view of a partially formed integratedcircuit structure having a thin film resistor formed in a recess in adielectric layer over a substrate;

FIG. 21 is a simplified isometric view of a thin film resistor laterallyconnecting adjacent interconnect structures according to an embodimentof the present disclosure; and

FIG. 22 is a top down view of the thin film resistor and theinterconnect structures of FIG. 21.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In someinstances, well-known structures associated with the manufacturing ofsemiconductor wafers have not been described in detail to avoidobscuring the descriptions of the embodiments of the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments.

In the drawings, identical reference numbers identify similar featuresor elements. The size and relative positions of features in the drawingsare not necessarily drawn to scale.

FIG. 3 is a cross-sectional view of a portion of an integrated circuit100 having a thin film resistor 102 laterally connecting a firstinterconnect structure 104 a and a second interconnect structure 104 bof a plurality of interconnect structures 104. Each interconnectstructure 104 includes a first conductive layer 106 and a secondconductive layer 124. The thin film resistor 102 connects the firstconductive layers 106 a, 106 b of the first and second interconnectstructures 104 a, 104 b. The integrated circuit 100 includes a substrate108 onto which a plurality of transistors, diodes, and other electronicdevices (not shown in this cross-section) are formed in conjunction withthe thin film resistor 102 to make the integrated circuit 100operational. The substrate 108 may be monocrystalline silicon,gallium-arsenide, or an alternative material onto which integratedcircuits are formed.

A first interlevel dielectric layer 110 is formed on the substrate 108to be used as insulation between transistors and other active componentsformed at other locations in the integrated circuit 100. The firstinterlevel dielectric layer 110 may be a deposited layer of oxide orother insulating material. For example, the first interlevel dielectricmay be a premetal dielectric layer (such as borophosphosilicate glass(BPSG)). A second interlevel dielectric 112 is formed over the firstinterlevel dielectric layer 110, which may be used to provide a topplanar surface 114 after the transistors or other components are formed.The second interlevel dielectric may be an insulating material, such astetraethyl orthosilicate (TEOS). The second interlevel dielectric 112also isolates the transistors or other components from a plurality offirst conductive structures 116. In one embodiment, the BPSG may be6,000 angstroms thick and the TEOS may be 16,000 angstroms thick.

Formation of the first conductive structures 116 is well known in theart and will not be described in detail. A variety of metals or otherconductive materials, such as aluminum, can be used to form the firstconductive structures 116. A third interlevel dielectric 118 isolatesthe plurality of first conductive structures 116 from each other andother devices formed in or over the substrate 108. The third interleveldielectric 118 may include a plurality of layers, such as multipledepositions of the same material or layers of different dielectricmaterials. The third interlevel dielectric 118 may be planarized afterformation by a chemical mechanical polish or other techniques thatremove irregularities from the surface of the integrated circuit 100.

A plurality of first conductive vias 120 are formed through the thirdinterlevel dielectric 118 to expose a top surface 122 of the firstconductive structures 116. The first conductive vias 120 can be formedof any conductive material, such as tungsten, copper, or aluminum, toprovide an electrical connection to the plurality of first conductivestructures 116. A barrier layer (not shown) may be formed as aprotective barrier to line the first conductive vias 120 prior toforming the conductive material in the first conductive vias 120. Forexample, the barrier layer may be titanium tungsten or titanium nitride.

The plurality of interconnect structures 104, including the first andsecond interconnect structures 104 a, 104 b, are formed over the thirdinterlevel dielectric 118. Methods of forming the plurality ofinterconnect structures 104 are described in more detail with respect toFIGS. 6-8. Each of the interconnect structures 104 have sidewalls 131that are transverse to a top surface 121 of the third interleveldielectric layer 118.

Each of the plurality of interconnect structures 104 includes a firstconductor 106 formed over the third interlevel dielectric 118 and asecond conductor 124 over the first conductor 106. An antireflectivecoating 126 overlies the second conductor 124. The antireflectivecoating 126 is optional and can be omitted. Each of the plurality ofinterconnect structures 104 have a protective coating 128, on theantireflective coating 126 or directly on the second conductor 124, thatforms a top surface 130 of the interconnect structures 104.

The thin film resistor 102 is formed by deposition of a thin filmresistor layer over the third interlevel dielectric 118 and theinterconnect structures 104. Numerous resistive materials may beutilized to form the thin film resistor 102, including, but not limitedto, metallic films like chromium silicon, nickel chromium, tantalumnitride, tantalum aluminum, and titanium nitride. These materials havebetter performance than conventional polysilicon resistors because theycan form a wide range of sheet resistances, have good tolerance, areeasily reproducible, and have low temperature coefficients ofresistance, have linear behavior, and have low parasitic capacitancevalues. These resistive materials are generally formed by an evaporationtechnique, a sputter technique, or a chemical vapor depositiontechnique.

Precise resistance control of the thin film resistor permits highquality analog circuits such as analog-to-digital converters anddigital-to-analog converters to be constructed. If careful selection isused in deciding the type of thin film resistor layer to use, a higherquality circuit can be constructed. The resistive value of the thin filmresistor 102 is called the sheet resistance, which is a measure ofresistance in thin films that have a uniform thickness. The sheetresistance of each thin film resistor depends on the length and width ofthe resistor, the material used to form the resistor, and the operatingtemperature of the associated integrated circuit or electronic device.The following formula is used to calculate sheet resistance, R, measuredin ohms per square (ohms/square).

R=ρL/wt

where ρ is the bulk resistivity, L is the resistor length, w is theresistor width, and t is the resistor thickness.

The thin film resistor 102 has a controllable sheet resistance and acontrollable temperature coefficient of resistance, which both depend onthe composition of the material and the process conditions. In someproducts, a temperature coefficient of resistance of zero is desired.For example, chromium silicon films have been developed for use inprecision integrated circuits, such as temperature sensors and currentsensors. Chromium silicon achieves a high sheet resistivity in the rangeof 2,000 and 3,000 ohms/square, which results in a high resistance in asmall area of the integrated circuit. Resistance of chromium siliconfilms depends on the percentage of silicon in the composition and can beeasily tailored to meet the resistance specifications of a particularintegrated circuit. Chromium silicon also exhibits a low temperaturecoefficient of resistance in the range of ±250 parts per million perdegrees Celsius (ppm/C) and can reach a near-zero temperaturecoefficient of resistance with specialized processing.

The thin film resistor 102 may be formed to have a thickness of lessthan 100 angstroms. In other embodiments, the thin film resistor 102 mayhave a thickness in the range of 50 and 500 angstroms. With these smallthicknesses, the thin film resistor 102 can be formed between adjacentinterconnect structures 104 without negatively impacting the interleveldielectric planarization of subsequent levels in the integrated circuit100.

The first and second interconnect structures 104 a, 104 b areelectrically connected to the thin film resistor 102 through the firstconductors 106 a and 106 b. This eliminates the need to form vias toconnect a thin film resistor to the next metal or conductive layer as inFIG. 1. Instead, the first conductors 106 a, 106 b allow the thin filmresistor 102 to be formed on the same level of the integrated circuit100 as the first and second interconnect structures 104 a, 104 b. Thissignificantly reduces manufacturing time and costs by reducing theprocessing steps and reducing the amount of materials used to completethe integrated circuit 100. Also, the overall vertical and horizontaldimensions of the integrated circuit are decreased.

In FIG. 3, vertical portions 132 of the thin film resistor layer remainon sidewalls of the interconnect structures 104. In some otherembodiments, the vertical portions 132 of the thin film resistor layerdo not remain in the final product. Their existence depends on how themanufacturer decides to pattern and form the thin film resistor 102 andhow to expose the top surface 130 of the interconnect structures 104. Ifthe vertical portions 132 of the thin film resistor layer are leftintact, these vertical portions 132 protect the interconnect structurefrom under etch when the thin film resistor 102 is defined and portionsof the third interlevel dielectric 118 are reexposed. Alternativestructures will be described in more detail below, with respect to FIGS.6-20.

The thin film resistor layer on the top surface 130 of the interconnectstructures 104 is partially or fully removed when defining the thin filmresistor 102. The protective coating 128 prevents the etch chemistryfrom damaging the second conductors 124 and the antireflective coating126 during the removal of the thin film resistor layer from the topsurfaces 130 of the interconnect structures 104. Some of the protectivecoating 128 may be removed during the etch; however, the protectivecoating 128 prevents damage to the second conductors 124 during removalof the excess thin film resistor layer.

In some embodiments, a surface 134 of the third interlevel dielectric118 is exposed when the thin film resistor 102 is patterned from thethin film resistor layer. As shown in FIG. 3, an etch to remove the thinfilm resistor layer may over etch and remove some amount of the exposedthird interlevel dielectric 118 to form the surface 134. The over etchmay occur to ensure the excess portions of the thin film resistor layerare removed, thereby avoiding shorting adjacent interconnect structures104 that are not intended to be electrically connected.

A fourth interlevel dielectric 136 is formed over the top surfaces 130of the interconnect structures 104, the thin film resistor 102, and thesurface 134 of the third interlevel dielectric 118. A plurality ofsecond conductive vias 138 extend through the fourth interleveldielectric 136 and the protective coating 128 to expose theantireflective coating 126 or the second conductor 124. As with theplurality of first conductive vias 120, the plurality of secondconductive vias 138 have a conductive material formed within in order toprovide an electrical connection from a plurality of second conductivestructures 140 to the interconnect structures 104. The second conductivestructures 140 are formed as is known in the prior art and will not bedescribed in detail herein.

A fifth interlevel dielectric 142 forms a top surface 144 of theintegrated circuit 100 as shown. However, additional metal layers andother structures can be formed as desired to make the integrated circuitoperational. Additional thin film resistors similar to the thin filmresistor 102 may be formed at other locations as desired.

FIG. 4 is a cross-sectional view of the integrated circuit 100 havingthe thin film resistor 102 covered by a dielectric cap 105. In oneembodiment, the thin film resistor 102 is chromium silicon and thedielectric cap 105 is silicon nitride. This combination results in thethin film resistor 102 having an ultra low temperature coefficient ofresistance of less than 10 ppm/C. The silicon nitride cap protects thechromium silicon from plasma etch steps. If left uncovered, the siliconin the chromium silicon can react with the oxygen in the etch chamberand change the resistance of the thin film resistor layer.

After the cap 105 is formed over the thin film resistor 102, a patternand etch are performed to form the desired thin film resistor 102. Asize and shape of the thin film resistor 102 are related to the desiredresistance value for the thin film resistor 102.

FIG. 5 is a cross-sectional view of the integrated circuit 100 having athin film resistor 102 having a first and a second thin film resistorlayer 103 a, 103 b. The thin film resistor 102 is covered by thedielectric cap 105 described above. Precise and reliable resistancevalues are reproducible by depositing multiple thin film resistor layersin succession. A stack of thin film resistor layers combine theelementary properties of the individual layers. The first and secondthin film resistor layers 103 a, 103 b may be deposited with multipledeposition steps in a single physical vapor deposition machine, withouta break in vacuum conditions.

FIGS. 6-11 are cross-sectional views of an integrated circuit 148 atvarious stages of a method of forming the interconnect structures 104and the thin film resistor 102. A first conductor layer 152 is formedover a substrate 150, which includes a partially formed integratedcircuit structure. Details of the integrated circuit structure are notshown because various components may be included. Similarly to theembodiments shown in FIGS. 3-5, the substrate 150 may include the firstand second interlevel dielectrics 110 and 112. The substrate 150 mayhave a plurality of transistors or active devices, metal layers, andinterlevel dielectric layers formed on a monocrystalline silicon chip.Alternatively, the substrate 150 may include all of the components ofFIGS. 3-5 that are formed prior to planarizing the third interleveldielectric 118. The substrate 150 may also include multiple metallayers. Formation of the thin film resistor 102 may be implemented in avariety of locations and at any metal level as needed in the manufactureof the electronic devices.

In one embodiment, the first conductor layer 152 is a 500 to 1,000angstrom layer of titanium. Titanium is sufficiently conductive to allowgood electrical connection between the first conductor 106 and the thinfilm resistor 102. The first conductor layer may be sputtered ordeposited over the substrate 150 to form a conformal layer. Otherconductive materials may be substituted for or combined with titanium,such as titanium nitride, titanium tungsten, chromium, tantalum nitride,and tantalum silicon nitride. In one embodiment, the first conductorlayer is 500 to 1,000 angstroms in thickness.

A second conductor layer 154 is formed overlying the first conductorlayer 152. The second conductor layer 154 may be formed using knownmetal formation techniques with materials such as aluminum, aluminumcopper alloys, copper, or other suitable conductive materials. Thesecond conductor layer 154 may be formed to have a thickness between2,000 angstroms and 1 micron. The second conductor layer 154 issignificantly larger than the first conductor layer 152. In oneembodiment, the first conductor layer 152 is a barrier that protects thesecond conductor layer 154 from diffusion from other elements in thesubstrate.

An antireflective coating layer 156 is formed overlying the secondconductor layer 154. The antireflective coating layer 156 is optionaland can be included depending on the type of metal used to form thesecond conductors 124. The antireflective coating layer 156 may be a 500angstrom thick layer of titanium nitride. Other suitable antireflectivecoatings may also be used.

A protective coating layer 158 is formed overlying the antireflectivecoating layer 156. A material for the protective coating layer 158 isselected to have different etch chemistry from the second conductor 154.For example, the protective coating layer 158 will be a dielectric ifthe second conductor layer 154 is metallic. This is to prevent an overetch or otherwise act as a stopping layer when future pattern and etchsteps threaten to damage the second conductor 154. The protectivecoating layer 158 may be a deposited layer of silicon dioxide, such as aTEOS layer in the range of 1,000 and 2,000 angstroms in thickness.Alternatively, the protective coating layer 158 may be a siliconnitride, silicon carbide, or other dielectric.

After the first conductor layer 152, the second conductor layer 154, andat least the protective coating layer 158 are formed over the substrate150, a photoresist pattern 160 is formed to define the interconnectstructures 104. The photoresist pattern 160 protects the interconnectstructures 104 as excess portions of the first conductor layer 152, thesecond conductor layer 154, the antireflective coating layer 156, andthe protective coating layer 158 are removed to reexpose a top surface162 of the substrate 150, see FIG. 7. The etch used to define theinterconnect structures 104 may overetch past the top surface 162 of thesubstrate 150 and form a recess as shown in FIGS. 14, 19, and 20.

The interconnect structures 104 can be between 5,000 angstroms and 1micron in thickness. After defining the interconnect structures 104,each interconnect structure 104 includes the first conductor 106, thesecond conductor 124, the antireflective coating 126, and the protectivecoating 128 as previously described in FIGS. 3-5.

In FIG. 8 a thin film resistor layer 164 is formed overlying theinterconnect structures 104 and the top surface 162 of the substrate150. In one embodiment, the thin film resistor layer 164 is a 50 to 500angstrom resistive film. However, other thicknesses are possible.

With stability in mind, materials with a low temperature coefficient ofresistance are selected, especially since many conductive materialschange resistance with changes in temperature. Temperature coefficientof resistance is the resistance-change factor per degree Celsius oftemperature change. A positive temperature coefficient of resistance fora material means that its resistance increases with an increase intemperature. Pure metals typically have positive temperature coefficientof resistance. Temperature coefficients of resistance approaching zerocan be obtained by alloying certain metals, and thus have negligiblevariation of temperature.

A negative temperature coefficient of resistance for a material meansthat its resistance decreases with an increase in temperature.Semiconductor materials, like carbon, silicon, and germanium, typicallyhave a negative temperature coefficient of resistance. Therefore,selection of the material to form the thin film resistor layer 164 willcontemplate the temperature coefficient of resistance in light ofexpected process conditions.

The thin film resistor layer 164 may be chromium silicon, platinum,titanium nitride, tantalum nitride, tantalum aluminum, or nickelchromium, to name a few. As mentioned above, the sheet resistance of thethin film resistor layer 164 depends on the material selected, thelength and width of the final resistor 102, and the operatingconditions. Variations in the chemical composition of the thin filmresistor layer also impact the sheet resistance.

For example, if using chromium silicon as the thin film resistor layer164 the amount of silicon can be varied to alter the resistance.Chromium silicon with 25% silicon can achieve 1 k ohms/square sheetresistance with a temperature coefficient of resistance of less than 100ppm/C. If the silicon is increased to 40%, then the chromium siliconlayer can achieve 10 k ohms/square sheet resistance with a temperaturecoefficient of resistance between 100 and 1,000 ppm/C. In addition, ifthe silicon content is 85%, a 100 k ohms/square sheet resistance with atemperature coefficient of resistance between 1,000 and 10,000 ppm/C canbe achieved.

More particularly, a chromium silicon resistor with 25% silicon has ahigher sheet resistance with a lower temperature coefficient ofresistance than other materials such as tantalum nitride and tantalumaluminum, which both have less than 0.1 k ohms/square with a temperaturecoefficient of resistance between 50 and 100 ppm/C. The chromium siliconresistor with 40% silicon has a higher sheet resistance of 10 kohms/square at a temperature coefficient of resistance of 500 ppm/C ascompared to a tantalum silicon nitride film, which has a resistancearound 0.1 k ohms/square. The chromium silicon with 85% silicon has ahigher resistance and a higher temperature coefficient of resistancethan a high-ohmic polysilicon resistor or a diffused resistor.

The thin film resistor layer 164 is deposited conformally over theinterconnect structures and the top surface of the substrate 150. Aphysical vapor deposition (PVD) technique or a PVD sputter technique maybe used to form the thin film resistor layer 164. For example, asputtering process, such as Magnetron Sputtering uses a sputter gas,such as argon krypton, supplied to a vacuum chamber. A sputter target,the cathode connected to a DC power supply, is negatively biased. As thecathode voltage increases, electrons are ejected from the sputtertarget's surface. Electrons collide with the argon atoms in the sputtergas to create Ar⁺ ions and more electrons. Rotation magnetic fields helpmaintain plasma by retaining electrons near the target surface.Electrons hop on the target surface area to ionize sputter gas. The Ar⁺ions are accelerated across the plasma sheath to kick out atoms from thetarget surface. Then the sputtered atoms travel across to the substratewhere they are deposited as the resistive film.

For example, the PVD sputter technique can be used to form a chromiumsilicon film using low power, such as 100 Watts with a process temperateof 350 degrees Celsius. Other temperature and power settings are alsosuitable. Low power is one factor in forming very thin films. In oneembodiment, the thin film resistor layer 164 is deposited with an argongas flow of 45 standard cubic centimeters per minute with a nitrogen gasflow of 2 standard cubic centimeters per minute for 40 to 50 seconds. Anincrease of nitrogen or oxygen in the deposition chamber results in morenegative temperature coefficients of resistance for the thin film.

In one embodiment, the thin film resistor layer has a resistance rangebetween 1 k and 2 k ohms/square. Thin film resistors formed inaccordance with the present disclosure can achieve a temperaturecoefficient of resistance in the range of zero to 100 ppm/C. A lowtemperature coefficient of resistance depends on the materialcomposition and the sheet resistance. For example, a thin film resistorlayer formed from a target of chromium boride (85%), silicon (10%), andsilicon carbide (5%) has a sheet resistance of around 2 k ohms/squarewith a temperature coefficient of resistance in the range of negative100 ppm/C to positive 150 ppm/C, depending on a thickness of the layer.This combination of materials have bi-modal grains sizes with smallgrains 3-5 nanometers in diameter and large grain sizes of 10 nanometersin diameter.

In another embodiment, a thin film resistor layer formed from a targetof chromium boride (55%), silicon (30%), and silicon carbide (15%) has asheet resistance of around 5 k ohms/square with a temperaturecoefficient of resistance around negative 420 ppm/C. This film'stemperature coefficient of resistance is weakly dependent on thickness.In yet another embodiment, a thin film resistor layer formed from atarget of chromium boride (35%), silicon (45%), and silicon carbide(25%) has a sheet resistance of around 25 k ohms/square with atemperature coefficient of resistance in the range of negative 1,800ppm/C and negative 1,500 ppm/C, depending on the thickness of the layer.

Both the thin film resistor layer properties and the depositionconditions impact the resistance and temperature coefficient ofresistance of the thin film resistor. Controlling the target's conditionand the deposition conditions contribute to obtaining a low temperaturecoefficient of resistance. Forming the thin film resistor layer 164laterally connecting sidewalls of adjacent interconnect structuresallows precise control of resistance and temperature coefficient ofresistance. For example, a high resistance in a small area, such as asheet resistivity in the range of 2,000 to 3,000 ohms/square, may beachieved.

A good electrical connection is achieved between the thin film resistorlayer 164 and the first conductors 106 in each interconnect structure104. The reduced thickness of the thin film resistor layer 164 and thelateral electrical connection allows a significant reduction inthickness of the integrated circuit 148.

After the thin film resistor layer 164 is formed, a dielectric cap layer165 is formed overlying the thin film resistor layer 164. The dielectriccap layer 165 may have a thickness in the range of 200 to 1,000Angstroms. The dielectric cap layer 165 is a dielectric that providesprotection and stability for the thin film resistor layer 164. Thedielectric cap layer 165 adds stability to the thin film resistor 102without increasing the size of the integrated circuit 148. Including thedielectric cap layer 165 over the thin film resistor layer 164 provideslong term resistance stability and generates improved voltagecoefficients.

In one embodiment, the thin film resistor layer 164 is a 50 to 100angstrom chromium silicon layer and the dielectric cap layer 165 is asilicon nitride cap layer in the range of 200 to 500 angstroms.Utilizing chromium silicon with silicon nitride achieves a stable sheetresistance with an ultra low temperature coefficient of resistance, suchas a temperature coefficient of resistance between negative 10 ppm/C andpositive 10 ppm/C. In other embodiments, a temperature coefficient ofresistance in the range of negative 250 ppm/C and positive 250 ppm/C canbe achieved. With specialized processing, a temperature coefficient ofresistance of zero can be achieved.

After the thin film resistor layer 164 is deposited, a hard mask may bedeposited to permanently protect the thin film resistor. For example, atitanium tungsten barrier layer may be deposited. The hard mask willprotect the thin film resistor from chemically reacting with subsequentinsulation or passivation layers. In locations where vias are formed,the hard mask can be removed with a wet etch, such as with hydrogenperoxide.

In FIG. 9, a photoresist pattern 166 defines the thin film resistor 102from the thin film resistor layer 164. The photoresist pattern 166covers a top surface 168 of the dielectric cap layer 165 over theinterconnect structures 104. The photoresist pattern 166 also covers thedielectric cap layer 165 over the thin film resistor 102 to be definedbetween the first interconnect structure 104 a and the secondinterconnect structure 104 b. The complexity of the masking can bereduced by only covering the desired thin film resistor 102.

A spacing between the interconnect structures 104 can be reduced as aresult of incorporating the protective coating 128 in the interconnectstructures 104. As the integrated circuit 148 is scaled to be smallerand smaller, the spacing between the interconnect structures is reduced.The challenge is to open the space between interconnect structures 104not connected by the thin film resistor 102. The protective coating 128allows reduced coverage by the photoresist pattern.

In FIG. 9, the photoresist pattern 166 extends to edges 170 formed byvertical portions of the thin film resistor layer 164 and the dielectriccap layer 165 along vertical portions of the interconnect structures104. Only horizontal portions 172 of the thin film resistor layer 164and the dielectric cap layer 165 are exposed to an etch intended todefine the thin film resistor 102 between the first and secondinterconnect structures 104 a, 104 b.

The photoresist pattern 166 could extend past edges 170 to cover anamount of the horizontal portions 172 of the thin film resistor layer164. As mentioned above, the challenge is to open and fully remove thethin film resistor layer 164 between interconnect structures 104 notconnected by the thin film resistor 102. When the photoresist pattern166 extends past the edges 170, opening the small spacing between theinterconnect structures becomes more challenging. Having the photoresistpattern 166 extend past the edges 170 risks shorting the thin filmresistor by not etching enough of the horizontal portions 172.

By incorporating the protective coating 128 in the interconnectstructures the photoresist pattern 166 can be formed to be flush withthe edges 170 or may only partially cover the top surface 168 of theinterconnect structures 104. The photoresist pattern 166 can beincorporated in less than sub-half-micron technologies and allow forsufficient removal of the horizontal portions 172 between adjacentinterconnect structures 104.

The etch to remove the horizontal portions 172 of the thin film resistorlayer 164 can also etch the second conductors 124. Depending on theseverity of the over etch, the integrity of the affected interconnectstructure may be compromised to a non-operational level. If theprotective coating 128 is not included, there is a risk that the secondconductors 124 will be damaged when the etch removes the thin filmresistor layer 164 from the top of the interconnect structures 104. Thisrisk is particularly acute when considering the difficulty in accuratelyand robustly stopping the etch, such as with endpoint detection. Withthe protective coating 128, any misalignment of the photoresist pattern166 or intentional partial exposure of the top surface 168 is not aproblem because the protective coating 128 will prevent the etch fromdamaging the second conductors 124. For example, using TEOS as theprotective coating 128 protects the second conductors 124 when removingthe thin film resistor layer 164 since a dielectric has a different etchchemistry than the thin film resistor layer.

The photoresist pattern 166 provides a minimum spacing betweeninterconnect structures 104 where the horizontal portions 172 of thethin film resistor layer 164 are desired to be removed. Even if maskmisalignment causes the photoresist pattern 166 to be located inwardfrom the edge 170, the interconnect structures will not be damaged bythe etch.

FIG. 10 is a cross-sectional view of the integrated circuit 148 afterthe etch to define the thin film resistor 102. The thin film resistorlayer remains intact over each interconnect structure 104 since thephotoresist pattern 166 extended to the edges 170. The etch to definethe thin film resistor 102 and to remove the horizontal portions 172 islong enough to fully remove the horizontal portions 172. If the etch isnot complete and some of the horizontal portions 172 remain, theintegrated circuit 148 may not function because the interconnectstructures 104 may remain electrically connected through the thin filmresistor 102. In order to ensure separation, the etch may be prolongedto over etch and expose the surface 134 of the substrate 150, which isbelow the top surface 162. By covering the thin film resistor 102 duringthe etch with the photoresist pattern 166, the small amount of spacebetween the interconnect structures 104 can be opened up.

The vertical portions 132 that remain after removing the horizontalportions 172 protect the interconnect structures 104 from being undercutduring the etch. A thickness of the vertical portions 132 may be reducedduring the etch to remove the horizontal portions 172.

The photoresist pattern 166 may be formed to only cover a portion of thetop surface 168 of the dielectric cap layer 165 over the thin filmresistor layer 164 that is over the first and second interconnectstructures 104 a, 104 b and to not cover the other interconnectstructures 104 not covered by the photoresist pattern 166. The etch toremove the horizontal portions 172 of the thin film resistor layer 164and the dielectric cap layer 165 will expose the surface 134 of thesubstrate 150. The etch will also remove the dielectric cap layer 165and the thin film resistor layer 164 not protected by the photoresistpattern 166 on the top of the interconnect structures 104 not associatedwith the thin film resistor 102. Some or all of the protecting coating128 may be removed from these other interconnect structures 104.

The antireflective coating 126 may be reexposed by this etch. Inembodiments where the antireflective coating 126 is omitted, thethickness of the protective coating 128 may be selected to preventdamage to the second conductor 124. If the photoresist pattern does notcover all of the interconnect structures, the uncovered interconnectstructures will not require additional processing before protecting themwith the fourth interlevel dielectric 136 and forming the plurality ofsecond conductive vias 138 (as shown in FIG. 3).

FIG. 11 is the integrated circuit 148 after removal of the dielectriccap layer 165 and the thin film resistor layer 164 that remained on theinterconnect structures 104 after removing the horizontal portions 172of the thin film resistor layer 164. Some or all of the protectivecoating 128 may be removed when thin film resistor layer 164 is removedfrom the top of the interconnect structures 104.

A top surface 133 of the protective coating 128 is exposed. This may beachieved in a variety of ways. For example, a dielectric layer (notshown) may be formed over the integrated circuit 148 and then a chemicalmechanical polish may be used to expose the top surface 133.Alternatively, the photoresist pattern 166 may be formed to only coverthe thin film resistor 102 such that the other top surface 133 isexposed when the horizontal portions 172 are removed.

The thin film resistor 102 connects adjacent interconnect structuresdirectly together without needing vias to connect the resistor to theinterconnects. This significantly reduces the space used to form theintegrated circuit 148. In addition, the space between interconnectstructures can be reduced when the protective coating 128 isincorporated in the interconnect structures.

A resistance of the thin film resistor 102 may be calculated by thefollowing equation:

R _(total)=2((R _(CS) //R _(top) //R _(side))+R _(angle))+R _(TF),

where R_(CS) is the resistance of the second conductor 124, R_(top) isthe resistance of a horizontal contact that forms the top surface 168 ofthe thin film resistor 102 on the first and second interconnectstructures 104 a, 104 b, and R_(side) is the resistance of a verticalcontact of the thin film resistor 102 adjacent sidewalls of the firstand second interconnect structures 104 a, 104 b. R_(angle) is theresistance at a contact angle between the thin film resistor 102 and thefirst conductors 106. R_(TF) is the resistance of the thin film resistor102. The equation determines the resistance of R_(CS) in parallel withR_(top) in parallel with R_(side). The overall resistance is mostimpacted by the R_(angle) between the first conductors 106 and the thinfilm resistor 102. When R_(CS)<<R_(top) and R_(LC), then R_(total) isapproximately equal to 2(R_(CS)+R_(angle))+R_(TF).

FIG. 12 is a cross-sectional view of the integrated circuit 148 havingan alternative photoresist pattern 167 formed over the thin filmresistor 102. The photoresist pattern 167 is intentionally formed inwardfrom the edge 170 on the top surface 168 of the interconnect structures104. In this circumstance, the protective coating 128 prevents the etchfrom damaging the second conductor 124 in the interconnect structures104 during an uncovered etch.

The photoresist pattern 167 may be formed to only cover a portion of thetop surface 168 of the thin film resistor layer 164 over the first andsecond interconnect structures 104 a, 104 b. The top surface 168 of theother interconnect structures 104 are not covered by the photoresistpattern 167. As described above, the protective coating 128 may be aTEOS layer or other silicon dielectric layer. The protective coating 128may also be a plurality of layers tailored to protect the secondconductors 124 and depending on the type of material used for the secondconductors 124.

FIG. 13 is a cross-sectional view of the integrated circuit 148 of FIG.12 after the thin film resistor 102 has been defined and the photoresist167 has been removed. As the etch removes the horizontal portions 172 ofthe thin film resistor layer 164, the surface 134 of the substrate 150is exposed and the thin film resistor layer 164 not protected by thephotoresist pattern 167 is removed from the top of the associatedinterconnect structures 104. Some or all of the protective coating 128may be removed from these interconnect structures 104.

A portion of the top surface 168 of the first and second interconnectstructures 104 a, 104 b exposed by the photoresist pattern 167 isremoved during the etch. Some or all of the protective coating 128 isalso removed during the etch and the antireflective coating 126 isreexposed. In embodiments where the antireflective coating 126 isomitted, the thickness of the protective coating 128 may be selected toprevent damage to the second conductor 124. In FIG. 13, the interconnectstructures do not require additional processing before protecting themwith the fourth interlevel dielectric 136 and forming the plurality ofsecond conductive vias 138 (as shown in FIG. 3).

FIG. 14 is an enhanced view of one of the interconnect structures 104electrically connected to the thin film resistor 102 through the firstconductor 106. The interconnect structure 104 includes the secondconductor 124 over the first conductor 162 and the antireflectivecoating 126 over the second conductor 124. Only a portion of theprotective coating 128 remains after the thin film resistor 102 isdefined from the first and second thin film resistor layers 103 a, 103b. A photoresist pattern similar to the photoresist pattern 167 in FIG.12 was used to define the thin film resistor 102.

In FIG. 14, the thin film resistor 102 is formed in a recess 210 in thesubstrate 150. The recess 210 is formed when the interconnect structures104 are formed over the substrate 150. The etch to define the individualinterconnect structures can over etch the substrate 150 and form therecess 210. The thickness of the first and second thin film resistorlayers 103 a, 103 b and the dielectric cap 105 are significantly lessthan a depth of the recess 210 below the top surface 162 of thesubstrate 150.

In all of the embodiments described herein, the thin film resistor maybe a single thin film resistor or a plurality of thin film resistorlayers. As described above, the formation of the thin film resistorlayers depend on the target material and gas and temperature conditionsof the deposition chamber. A plurality of layers having differentchemical compositions can be deposited to achieve a desired resistancevalue for the thin film resistor. For example, the first resistor layer103 a may be a chromium silicon film having a negative temperaturecoefficient of resistance, such as −360 ppm/C. The second resistor layer103 b may be another chromium silicon film having a positive temperaturecoefficient resistance, such as +400 ppm/C. A resistance value of thethin film resistor can be determined by calculating the parallelresistance values of the first and second resistor layers 103 a, 103 b.A near zero temperature coefficient of resistance can be achieved byforming the first resistive layer having the negative temperaturecoefficient resistance and the second resistive layer the positivetemperature coefficient resistance.

In one embodiment, the first resistive layer 103 a has a thickness inthe range of 100 and 150 angstroms. The second resistive layer 103 b hasa thickness in the range of 20 and 50 angstroms. The first resistivelayer 103 a acts as a liner, electrically coupling the second resistivelayer 103 b to the interconnect structures. The first resistive layer103 a provides better continuity for the lateral contact. The first andsecond resistive layer 103 a, 103 b may be alloys formed from the sameelements, but having different resistance values and differenttemperature coefficients of resistance caused by variations indeposition techniques. Alternatively, the first and second resistivelayers may be alloy compositions that are formed from differentelements.

The first and second resistive layers 103 a, 103 b may be variousthicknesses and have various sheet resistances. The vertically stackedarrangement of the first and second resistive layers is a parallelresistor structure. The deposition target and gas conditions areselected for each resistive layer so that the overall resistance of theplurality of resistive layers in parallel equals a desired resistancevalue for the thin film resistor.

In one embodiment, the target for first resistor layer 103 a could bechromium boride (85%), silicon (10%), and silicon carbide (5%). Thetarget for the second resistor layer 103 b could be chromium boride(55%), silicon (30%), and silicon carbide (15%). A third resistor layer(not shown) could be formed on the second resistor layer 103 b from atarget of chromium boride (35%), silicon (45%), and silicon carbide(25%). The combination of the first, second, and third resistor layersforms the thin film resistor 302. In one embodiment, the thin filmresistor having three resistive layers formed in accordance with theabove referenced targets has a sheet resistance of 200 ohms/square and atemperature coefficient of resistance near zero ppm/C.

The plurality of thin film resistor layers can be deposited with asingle machine without disrupting vacuum conditions. The plurality offilms will compensate each other to achieve desired electricalproperties. For example, a film with a negative temperature coefficientof resistance can be covered with a film having a positive temperaturecoefficient of resistance. The combination of the positive and negativetemperature coefficients of resistance provide for near zero temperaturecoefficient of resistance values. The plurality of thin film resistorlayers provide for stable sheet resistances of less than one percentvariation across a wafer.

FIGS. 15-18 are cross-sectional views of another embodiment of the thinfilm resistor 102 laterally connecting sloped interconnect structures174 over the substrate 150. In FIG. 15, the sloped interconnectstructures 174 include a titanium conductor layer 176 and a metal layer178. The titanium conductor layer 176 may be formed directly on thesubstrate 150 or spaced from the substrate by a dielectric layer (notshown). In an alternative embodiment, the titanium conductor layer mayinclude a titanium layer and a titanium nitride layer. As describedabove, other materials besides titanium may be used for the conductorlayer 176.

The metal layer 178 may be aluminum, copper, or other metallicmaterials. For example, the metal layer 178 may have an aluminum coppersilicon composition. The sloped interconnect structures 174 have angledsidewalls such that the interconnect structures are trapezoidal, i.e.,tapered toward an upper surface 180. The optimized profile of theinterconnect structures is a compromise between the vertical and slopedsidewalls in FIGS. 3 and 15, respectively.

Having an angled sidewall of the conductor layer 176 can enhance theperformance of the thin film resistor 102 between the slopedinterconnect structures 174. The thin film resistor 102 is formed from athin film resistor layer 164 in accordance with the methods andcompositions described above. The dielectric cap layer 165 is formedover the thin film resistor layer 164 for stability.

In FIG. 16, a photoresist pattern 184 is formed over the slopedinterconnect structures 174 such that a portion of the upper surface 180is covered. The photoresist pattern 184 is inward from an edge 187 ofthe dielectric cap layer 165, which begins the slope down towardhorizontal portions 186. Alternatively, the photoresist pattern 184 mayextend to the edge 187 of the dielectric cap layer 165. Having thephotoresist pattern 184 extend over the full top surface 180 of theinterconnect structures 174 provides protection for the top surface.Accordingly, the protective coating 128 described above, can be omitted.

FIG. 17 is the cross-sectional view of the thin film resistor 102 afterthe etch removes the horizontal portions 186 not protected by thephotoresist pattern 184. Vertical portions 188 of the thin film resistorlayer 164 are completely removed if the photoresist pattern 184 onlycovers the upper surface 180 of the sloped interconnect structures 174.However, if the photoresist pattern 184 extends past the upper surface180, portions or all of the vertical portions 188 may remain onsidewalls 190 of the sloped interconnect structures 174.

The thin film resistor 102 electrically connects the sloped interconnectstructures 174 through the conductor layer 176. This lateral connectionprovides the thin film resistor on the integrated circuit without theextra interlevel dielectrics and vias typically used for precisionresistors. Therefore the overall dimension of the integrated circuit isreduced.

FIG. 18 is the thin film resistor 102 with the cap 105 after thephotoresist is removed. Various interlevel dielectrics and vias may beformed to couple the thin film resistor to other interconnect structuresin the integrated circuit, as shown in FIG. 3. Upper horizontal portions192 can be removed or vias can be formed through the upper horizontalportions 192 to electrically connect the interconnect structures toother components in the integrated circuit.

FIG. 19 is yet another embodiment of an integrated circuit 200 having athin film resistor 202 with a dielectric cap 205. The integrated circuit200 includes a substrate 250 that may have a plurality of active devicesformed therein. A plurality of sloped interconnect structures 204 areformed over the substrate 250. Each interconnect structure 204 includesa first conductor 206 and a second conductor 208 and has sidewalls thatare transverse to a top surface 214 of the substrate 250. During theformation of the interconnect structures recesses 210 are formed in thesubstrate 250 between the interconnect structures 204. The recesses 210may be formed by over-etching the interconnect structures and by removalof the photoresist.

Subsequently, the thin film resistor 202 is deposited and covered withthe dielectric cap 205. The thin film resistor 202 has a top surface 212below the top surface 214 of the substrate 250. The dielectric cap 205also has a top surface 216 below the top surface of the substrate 250.The thin film resistor 202 and the dielectric cap 205 are significantlysmaller than the interconnect structures and smaller than the firstconductors 206.

FIG. 20 is a cross-sectional view of an integrated circuit 300 having athin film resistor 302 connecting first and second interconnectstructures 304 a, 304 b. The thin film resistor 302 is protected fromsubsequent processing steps by a dielectric cap 305. The integratedcircuit 300 includes a substrate 350, which may include a plurality ofactive devices, such as CMOS and bipolar transistors. A top surface 314of the substrate 350 may be may be a planarized layer of dielectricmaterial formed over the transistors or other metal levels.

Over the substrate 350, a first dielectric layer 310 is formed. Thefirst dielectric 310 may be a silicon nitride layer or other dielectricmaterial. A plurality of interconnect structures 304, including firstand second interconnect structures 304 a, 304 b, are formed over thefirst dielectric layer 310. An etch used to define the interconnectstructures 304 may overetch the first dielectric layer 310 and form arecessed surface 312.

The interconnect structures 304 include a first conductor 306 over thefirst dielectric layer 310 and a second conductor 308 over the firstconductor 306. A protective coating 328 is formed over the secondconductor 308. The protective coating 328 prevents the second conductor308 from damage during an etch to define the thin film resistor 302. InFIG. 20, a photoresist pattern (not shown) was used that fully covered atop surface of the thin film resistor 302 and the dielectric cap 305over the first and second interconnects 304 a, 304 b. The otherinterconnect structures 304, adjacent the first and second interconnectstructures 304 a, 304 b were not protected by the photoresist pattern.The protective coating 328 prevents the etch that removes the excessthin film resistor 302 from damaging the second conductors 308.

The thin film resistor 302 and the dielectric cap are formed over theinterconnect structures 304 and over the recessed surface 312 of thefirst dielectric layer 310. Accordingly, a top surface 316 of thedielectric cap 305 is below a top surface 315 of the first dielectriclayer 310. The thin film resistor 302 is formed to have a first portion320 in contact with the recessed surface 312 of the first dielectriclayer 310 between the first and second interconnect structures 304 a,304 b. The thin film resistor 302 also has second portions 322 incontact with sidewalls of the first and second interconnect structures304 a, 304 b.

The configuration of the thin film resistor 302 between the firstdielectric layer 310 and the dielectric cap 305 assists in heatdissipation, thereby making the film more stable. The heat is propagatedfrom the thin film resistor to the first and second conductors, avoidingtemperature extremes that can be caused by the joule effect. This alsoresults in more stable current values per changes in operatingtemperature of the thin film resistor 302. In one embodiment, the firstdielectric 310 and the dielectric cap 305 are silicon nitride.

FIG. 21 is a simplified isometric view of a partial integrated circuit400 having a first interconnect structure 404 a spaced from a secondinterconnect structure 404 b formed on a planar surface 421 of adielectric layer 410. The dielectric layer 410 may be formed over asubstrate that has active and passive devices formed thereon.

A thin film resistor 402 laterally connects sidewalls 431 of the firstand second interconnect structures 404 a, 404 b together. The first andsecond interconnect structures 404 a and 404 b each include a firstconductive layer 406 a, 406 b and a second conductive layer 424 a, 424b, respectively.

The first and second interconnect structures 404 a, 404 b extend acrossthe planar surface 421 to connect various electronic components formedin and over the substrate. The thin film resistor 402 is formed over aportion of the substrate between the first and second interconnectstructures 404 a, 404 b. The size of the portion of the substratecovered by the thin film resistor will vary depending on the desiredresistive value and characteristics of the resistive material used toform the thin film resistor.

FIG. 22 is a top down view of the partial integrated circuit 400 of FIG.21. The first interconnect structure 404 a extends between a firstcontact 405 and a second contact 407, which are spaced from each otheron the integrated circuit. The first contact 405 may couple to an uppermetal level in the integrated circuit 400. For example, the firstcontact 405 may connect to one of the plurality of first conductive vias120 in FIG. 3 that connects the first interconnect structure 104 a toone of the second conductive structures 140. The second contact 407 maycouple to an active element at a level below the first interconnectstructure 404 a. For example, the active element may be a transistor ora first metal level of the integrated circuit, such as metal one.

The second interconnect structure 404 b extends between a third contact409 and a fourth contact 411, which are spaced from each other on theintegrated circuit 400. As with the first and second contacts 405, 407of the first interconnect structure 404 a, the third and fourth contacts409, 411 connect the second interconnect structure to other elements ofthe integrated circuit 400. The third contact 409 couples the secondinterconnect structure 404 b to an element (not shown) that is above thesecond interconnect structure in the integrated circuit 400. The fourthcontact 411 couples the second interconnect structure 404 b to anotherelement (not shown) that is below the second interconnect structure 404b on the integrated circuit 400.

The thin film resistor 402 is formed to abut sidewalls 431 of the firstand second interconnect structures 404 a, 404 b, electrically connectingthe interconnect structures together. In one embodiment, the thin filmresistor 402 is formed adjacent the first contact 405 and the thirdcontact 409 of the first and second interconnect structures 404 a, 404b, respectively.

An alternative embodiment, not shown in the figures, uses additionalprocess steps to obtain a first conductor that is wider than the secondconductor so that more of the thin film resistor is in contact with thefirst conductor. The first conductor could form a step having a topsurface and a sidewall onto which the thin film resistor may be formed.Accordingly, the thin film resistor would have direct electrical contactwith the top and sidewall instead of only the sidewall.

Advantages of the uncovered etch of the thin film resistor layer to formthe thin film resistor include overcoming mask overlay process marginconstraints and allows for integration of the thin film resistor intoexisting products with minimal space between interconnect structures.Prior art methods use a mask for the resistor and for the metal lines,with an overlap. This method improves process margins and robustness formasking because the manufacturer only needs to mask thin film resistorareas. The simpler masking reduces time and material costs. Alsomanufacturing constraints based on choosing thin film materials to haveeither good etch selectivity to metal or to the antireflective coatingor to have observable endpoint trace differences for endpoint detectionare removed.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent application, foreign patents, foreign patentapplication and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, application and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A thin film resistor, comprising: a first resistor layer having afirst temperature coefficient of resistance; and a second resistor layeron the first resistor layer, the second resistor layer having a secondtemperature coefficient of resistance different from the firsttemperature coefficient of resistance.
 2. The thin film resistor ofclaim 1 wherein the first resistor layer and the second resistor layerform the thin film resistor having a third temperature coefficient ofresistance that is in the range of positive 20 parts per million perdegrees Celsius and negative 20 parts per million per degrees Celsius.3. The thin film resistor of claim 1 wherein the first temperaturecoefficient of resistance is positive and the second temperaturecoefficient of resistance is negative.
 4. The thin film resistor ofclaim 1 wherein the first resistor layer has a thickness in the range of50 and 150 angstroms and the second resistor layer has a thickness inthe range of 20 and 50 angstroms.
 5. The thin film resistor of claim 1wherein the first resistor layer is less than 50 angstroms in thicknessand the second resistor layer is less than 50 angstroms in thickness. 6.The thin film resistor of claim 1, further comprising a third resistorlayer on the second resistor layer and the third resistor layer having afourth temperature coefficient of resistance.
 7. A method, comprising:forming a thin film resistor on a substrate, the forming comprising:forming a first resistor layer on the substrate, the first resistorlayer having a first composition of a plurality of elements and having afirst thickness less than 200 angstroms; and forming a second resistorlayer on the first resistor layer, the second resistor layer having asecond composition of the plurality of elements and having a secondthickness that is less than 150 angstroms.
 8. The method of claim 7further comprising forming the first composition from a first target ina sputter deposition chamber and forming the second composition from asecond target in the sputter deposition chamber.
 9. The method of claim8 wherein the first target is 55% chromium and the second target is 85%chromium.
 10. The method of claim 7 wherein the first and secondcompositions are different alloys of chromium silicon.
 11. The method ofclaim 7 wherein the first resistor layer has a first temperaturecoefficient of resistance and the second resistor layer has a secondtemperature coefficient of resistance.
 12. The method of claim 10wherein the first temperature coefficient of resistance is positive andthe second temperature coefficient of resistance is negative.
 13. Themethod of claim 7, further comprising forming a third resistor layer onthe second resistor layer, the third resistor layer having a thirdcomposition of the plurality of elements.
 14. An integrated circuit,comprising: a substrate; a first interconnect having a first conductorlayer on the substrate and a second conductor layer on the firstconductor, the first interconnect having a first sidewall transverse tothe substrate; a second interconnect spaced from the first interconnect,the second interconnect having the first conductor layer on thesubstrate and the second conductor layer on the first conductor layer,the second interconnect having a second sidewall transverse to thesubstrate; a resistive element having a plurality of resistive layersformed on the substrate between the first and second interconnects andon the first and second sidewalls of the first and second interconnects,the plurality of resistive layers comprising: a first resistive layerelectrically connecting the first conductor layers of the first andsecond interconnects; and a second resistive layer formed on the firstresistive layer.
 15. The integrated circuit of claim 14 wherein thefirst resistive layer has a first temperature coefficient of resistanceand the second resistive layer has a second temperature coefficient ofresistance different than the first temperature coefficient ofresistance.
 16. The integrated circuit of claim 14 wherein the firstresistive layer and the second resistive layer are different chromiumsilicon alloys.
 17. The integrated circuit of claim 14 wherein the firsttemperature coefficient of resistance is positive and the secondtemperature coefficient of resistance is negative.